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# Fire Flow Test w Pitot Pressure

## Fire Flow Test w Pitot Pressure

(OP)
All,
Its been a while for me but for fire flows in the past I was used to receiving my static pressure, flow and residual pressure. I would then either graphically or mathematically would utilize these numbers to predict how the system performed to say either at minimum pressure of 25 or 20 psi.

I've received a flow test that includes a Static of 44 psi, residual pressure of 25 psi and a pitot pressure of 11 psi.

Using Q = 29.83 x c x d^2 x p^0.5 I'm arriving at 556 gpm as the flow (assuming c=0.9 and d=2.5 inches)

This gets me the flow of 556 gpm with a residual pressure of 25 psi.

Projecting this using your typical logarithmic graph for water flow and pressure, this results in about 630 gpm when the pressure goes down to 20 psi in the system.

Am I approaching this correctly? In short, I'm using the pitot pressure to arrive at my residual flow. In other words the pitot pressure is not necessarily the same as the system residual pressure.

### RE: Fire Flow Test w Pitot Pressure

I think you are using the data correctly, but those are some sad system numbers if you're trying to achieve any sort of fire flow.

#

### RE: Fire Flow Test w Pitot Pressure

(OP)
Thanks Pinwards

Yes, agree. Luckily there appears to be a newer and separate larger line which is exhibiting larger flows, in the order of 1750 gpm at 20 psi residual. So trying to clarify how older hydrants and the newer line are related by testing at different points. The data I provided was only for one of the three hydrants tested in the vicinity.

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